To date, for piping paths required to have chemical resistance or the like, tubes or pipe joints made of synthetic resin such as fluororesin have been used. As such, the pipe joint includes: a joint body to be mounted to a leading end portion of a tube; and a union nut having a female screw portion to be tightened onto a male screw portion formed on the outer circumferential portion of the joint body, is often used. The pipe joint has a structure in which the union nut is tightened onto the outer circumferential portion of the joint body, to assuredly obtain a sealing performance between the joint body and the tube.
For example, in a joint made of resin as described in Patent Literature 1, an inner ring (fitting portion) 13 is pressed into a leading end portion of a tube (tubing) 12, made of synthetic resin, in a pipe joint made of synthetic resin such that a fluid path is formed by an inner circumferential portion of the inner ring 13, and the inner ring 13 has a protrusion 15 that protrudes radially outward, as shown in FIG. 1 of Patent Literature 1. The tube 12, which has been deformed so as to have its diameter enlarged by the protrusion 15 of the inner ring 13 being pressed into the tube 12, is inserted into a receiver opening 4 of a joint body 1. The pipe joint made of synthetic resin includes the joint body 1 made of synthetic resin and a union nut (pressing ring) 3 made of synthetic resin. The joint body 1 is mounted on the outer circumferential side of the tube 12. The union nut 3 is mounted on the outer circumferential side of the joint body 1, and has a female screw portion 19 that is formed thereinside so as to be screwed onto a male screw portion 11 formed on the outer circumferential portion of the joint body 1. The pipe joint made of synthetic resin allows the union nut 3 to be screwed forward onto the joint body 1, and the union nut 3 to press, in the axis direction, the tube 12 into which the inner ring 13 is pressed, thereby providing connection to the tube 12.
In the above structure, in order to assuredly obtain a favorable sealing performance, a degree of tightening of the union nut onto the joint body needs to be controlled so as to represent an appropriate value. As a method for controlling a degree of tightening, for example, a method for tightening the union nut while measuring a tightening torque with the use of a torque wrench, a method for tightening the union nut by turning the union nut a predetermined number of times while measuring the number of turnings for tightening, a method for gradually tightening the union nut while repeatedly measuring a tightening extent with the use of a feeler gauge or the like, or a method in which a ring-shaped tightening regulation member is additionally provided for regulating a tightening movement when the union nut has been tightened to an appropriate degree (see, for example, Patent Literature 2), are known.